1. Field of the Invention
The present invention relates to a thin film solar cell and a method of manufacturing the same.
2. Description of Related Art
Today, future aggravation of supply and demand of petroleum and the like that have been utilized as a main energy source is concerned. The use of petroleum involves generation of CO2 which causes global warming, so that solar cells have drawn attention as an alternative energy source.
The solar cells employ a semiconductor pn junction as a photoelectric conversion layer for converting optical energy into electric power and silicon is mainly utilized as a semiconductor material comprising the pn junction.
Crystalline silicon solar cells utilizing materials including monocrystalline silicon and the like are advantageous in photovoltaic conversion efficiency and have already been put into practical use. However, they are problematic in that material supply is not stable, a cell area is insufficient and costs are high.
Further, according to eager researches and developments in recent years, amorphous solar cells utilizing amorphous silicon which have been practically utilized as the solar cells in consumer devices are now getting into practical use for power generation since they are advantageous to realize large area and low costs.
However, the photovoltaic conversion efficiency of the amorphous solar cells is as low as about 10%, which has not yet reached the photovoltaic conversion efficiency of 15 to 20% obtained by the crystalline solar cells. Further, amorphous silicon causes a phenomenon called Staebler-Wronski effect which increases defect density in the film as being irradiated with light. Accordingly, decrease of the photovoltaic conversion efficiency as time elapse is inevitable.
In connection with the above, researches and developments have eagerly been conducted in recent years with respect to solar cells utilizing a thin film of crystalline silicon, e.g., polycrystalline silicon and microcrystalline silicon, which exhibits high reliability and high photovoltaic conversion efficiency of the crystalline silicon solar cells as well as good efficiency in material consumption, large area and low costs of the amorphous silicon solar cells.
Plasma enhanced CVD method is one method of manufacturing a thin crystalline film to be used in the solar cells. In particular, a thin crystalline silicon film formed on a glass substrate by plasma enhanced CVD method at a low temperature not higher than 600° C. has drawn attention because experiences that have gone through in the manufacture of the amorphous solar cells are utilized.
However, the photovoltaic conversion efficiency of the solar cell manufactured with use of the thin crystalline silicon film prepared in the above method is on the same level as that of the solar cell of amorphous silicon.
The reason of the low photovoltaic conversion efficiency of the solar cell using the thin crystalline silicon film is mainly a low crystal fraction and an insufficient crystal orientation of the thin polycrystalline silicon film formed on a substrate of other material than silicon.
It is known that the crystal fraction is increased by selecting a formation temperature or increasing the ratio of hydrogen for diluting silane in a reaction gas as described in Thin Solid Films, vol. 337 (1999), p 1, for example.
It is also known that the crystal orientation is improved by selecting a suitable pressure and a composition of the reaction gas or adding particular elements.
For example, according to documents (“Growth and Characterization of Polycrystalline Silicon” J. Electrochem. Soc. SOLID-STATE SCIENCE AND TECHNOLOGY, Vol. 120(1973), No. 12, p. 1761 and “Chemical Vapor Deposited Polycrystalline Silicon” J. Electrochem. Soc. SOLID-STATE SCIENCE AND TECHNOLOGY, Vol. 119(1972), No. 11, p. 1565), a thin silicon film having a preferential crystal orientation in the direction of <110> is obtained by thermal CVD method using silane added with diborane at 680° C. and 650° C., respectively. Further, according to a document (Conferences of Japanese Society of Applied Physics, Autumn 1999, Summary 1p-ZS-2, p. 787), the thin silicon film having the preferential crystal orientation in the direction of <110> is obtained by plasma enhanced CVD method using silane added with diborane at about 200° C.
Further, a document (Conferences of Japanese Society of Applied Physics, Autumn 1999, Summary 2p-ZM-9, p. 32) has reported that the preferential crystal orientation in the directions <110> and <100> of the thin silicon film is controlled by plasma enhanced CVD method at 200 to 300° C. while changing the ratio between a material gas of silicon fluoride (SiF4) and a diluent gas of hydrogen.
Further, it is known that where the ratio of hydrogen for diluting silane in the reaction gas is relatively high, the crystal fraction becomes high and the crystal orientation in a specific direction becomes weak. On the other hand, where the ratio of diluent hydrogen is relatively low at the same temperature as the above, the crystal fraction becomes low but the crystal orientation in the direction of <110> becomes high.
Concretely speaking, even if the high crystal fraction is obtained by adjusting the conditions for forming the intrinsic photoelectric conversion layer, the ratio of integrated intensity of an X-ray diffraction peak at (220) with respect to an X-ray diffraction peak at (111) is equal or less than 2 when it is deposited to a thickness of about 500 nm as a single layer. Further, even if the high crystal orientation that allows the ratio of the integrated intensity of the X-ray diffraction peak at (220) with respect to that at (111) of 5 or more, the crystal fraction thereof will be reduced. The crystal properties of these thin crystalline silicon films are unsatisfactory to be used as the intrinsic photoelectric conversion layer for the solar cells.
In connection to the above, there has been proposed a technique in which a conductive layer formed under the intrinsic photoelectric conversion layer is utilized as a layer for controlling the crystal properties of the intrinsic photoelectric conversion layer (e.g., see Japanese Unexamined Patent Publication No. Hei 11 (1999)-145498).
The Publication refers to a thin conductive film of microcrystalline silicon employed as the conductive layer directly below the intrinsic photoelectric conversion layer. The impurity concentration which determines a conductivity type of the thin conductive film of microcrystalline silicon is in the range of 0.05 to 9 atom % and the thickness thereof is in the range of 1 to 10 nm or 1 to 30 nm. The crystal properties of the photoelectric conversion layer are enhanced by forming it on the thus constructed conductive layer.
However, in the Publication, the conductive layer of n-type is considered in detail, but the conductive layer of p-type is exemplified in only one example. Therefore, characteristics of the p-type conductive layer suitable as the layer for controlling the crystal properties of the intrinsic photoelectric conversion layer and a construction of the thin film solar cell using the p-type conductive layer are not satisfactorily explained.
The conductivity type of the layer formed at the bottom of the intrinsic photoelectric conversion layer greatly influences the structure of the solar cell. In comparison between electrons which are majority carriers in the n-layer and holes which are majority carriers in the p-layer, the holes are poorer in mobility and life. Therefore, it is well known that greater photovoltaic conversion efficiency is advantageously obtained by arranging the p-layer to receive light.
Accordingly, where the n-layer is formed at the bottom of the intrinsic photoelectric conversion layer, the solar cell employs in general a substrate type structure in which light enters from the opposite side to the substrate. In the substrate type structure, the photoelectric conversion element is located to be irradiated with light so that a surface protection is additionally required to prevent deterioration of the element due to exposure to air. The surface protection is often formed of a film of acrylic resin or fluoric resin. However, problems of the reduction of transmissivity due to denaturation by light irradiation for a long term, insufficient moisture-resistance and the generation of ridging on the surface are still unsolved.
On the other hand, where the p-layer is formed at the bottom of the intrinsic photoelectric conversion layer, and then a transparent substrate such as a glass substrate is utilized, the solar cell structure of a superstrate type can be employed in which light enters from the transparent substrate. In particular, where the glass substrate is used, the problems of reduced transmissivity and low moisture-resistance are solved. That is, it is more preferable to apply the solar cell of the superstrate type structure to a solar electric power generator for domestic use which is installed outside.
Thus, for wide use of the solar cell, it is extremely important to make sure of favorable characteristics of the p-layer which functions as a layer controlling the crystal properties of the intrinsic photoelectric conversion layer and a favorable structure of the thin film solar cell utilizing the p-layer as the bottom layer of the intrinsic photoelectric conversion layer.